Semiconductor device including gate electrode for applying tensile stress to silicon substrate, and method of manufacturing the same

ABSTRACT

A gate insulating film and a gate electrode of non-single crystalline silicon for forming an nMOS transistor are provided on a silicon substrate. Using the gate electrode as a mask, n-type dopants having a relatively large mass number (70 or more) such as As ions or Sb ions are implanted, to form a source/drain region of the nMOS transistor, whereby the gate electrode is amorphized. Subsequently, a silicon oxide film is provided to cover the gate electrode, at a temperature which is less than the one at which recrystallization of the gate electrode occurs. Thereafter, thermal processing is performed at a temperature of about 1000° C., whereby high compressive residual stress is exerted on the gate electrode, and high tensile stress is applied to a channel region under the gate electrode. As a result, carrier mobility of the nMOS transistor is enhanced.

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 14/325,570 filed Jul. 8, 2014,which is a continuation of U.S. Ser. No. 14/038,981 filed Sep. 27, 2013(now U.S. Pat. No. 8,809,186 issued Aug. 19, 2014), which is acontinuation of Ser. No. 13/743,012 filed Jan. 16, 2013 (now U.S. Pat.No. 8,586,475 issued Nov. 19, 2013), which is a continuation of U.S.Ser. No. 13/103,558 filed May 9, 2011 (now U.S. Pat. No. 8,372,747issued Feb. 12, 2013), which is a continuation of U.S. Ser. No.12/323,507 filed Nov. 26, 2008 (now U.S. Pat. No. 7,960,281 issued Jun.14, 2011), which is a continuation of U.S. Pat. No. 7,470,618 issuedDec. 30, 2008, which is a continuation of U.S. Pat. No. 7,183,204 issuedFeb. 27, 2007, which is a division of U.S. Pat. No. 6,906,393 issuedJun. 14, 2005, and claims the benefit of priority under 35 U.S.C. §119from Japanese Patent Application No. 2002-336669 filed Nov. 20, 2002,the entire contents of each of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor including a MOS (metaloxide semiconductor) field effect transistor, and method ofmanufacturing the same.

2. Description of the Related Art

For a MOS field effect transistor (MOS transistor), increase in draincurrent as a driving current is one of the ways of improvingcharacteristic of this MOS transistor. Carrier mobility is one of thedeterminants of drain current. The carrier mobility is virtuallycontrolled by a substrate material, and therefore, it can hardly bechanged. On the other hand, it has been found that scatteringprobability and effective mass of carriers are altered by the change inlattice spacing of substrate atoms, allowing change of carrier mobility.

SiGe has wider lattice spacing than Si. In a substrate including SiGeand Si stacked thereon, the lattice spacing of the upper-layer Si iswidened accordingly. The substrate including the widened lattice spacingof silicon is called as a “strained silicon substrate”. The strainedsilicon substrate has a higher carrier mobility than a conventionalsilicon substrate, providing increase in drain current of a MOStransistor formed thereon. An example of such conventional art is givenin the non-patent document 1, Welser et al., “NMOS and PMOS TransistorsFabricated in Strained Silicon/Relaxed Silicon-Germanium Structures”,pp. 1000-1002, International Electron Device Meeting 1992, and in thenon-patent document 2, T. Mizuno et al., “High Performance Strained-Sip-MOSFETs on SiGe-on-Insulator Substrates Fabricated by SIMOXTechnology”, pp. 934-936, International Electron Device Meeting 1999.

On the other hand, the strained silicon substrate encounters theproblems as follows which result from use of SiGe as a substratematerial: crystal defect and deterioration in surface roughness causedby SiGe, rise in substrate temperature due to low heat conductivity ofSiGe, increase in short-channel effect in a p-channel MOS transistorcovering band discontinuity at an interface between SiGe and Si, or thelike. Other problems involved therein associated with process stepsinclude inapplicability to STI (shallow trench isolation) technique, orinsufficient activation annealing, for example. In view of this, foractually using the strained silicon substrate in LSIs, there remain alot of problems to be solved.

By way of example, Japanese Patent Application Laid-Open No. 2002-93921(pp. 3-6 and FIGS. 1-19), hereinafter referred to as the patent document1, discloses that the lattice spacing of silicon of a MOS transistor maybe varied by applying stress to a silicon substrate.

By way of example, tensile stress exerted on a channel region causesincrease in driving current of an n-channel MOS transistor (NMOStransistor), while causing reduction in driving current of a p-channelMOS transistor (PMOS transistor). Conversely, compressive stress exertedon the channel region causes increase in driving current of the pMOStransistor, while causing reduction in driving current of the nMOStransistor.

As discussed, the strained silicon substrate including SiGe still facesthe problems to be solved. Therefore, more simple way has been sought toimprove characteristic of a MOS transistor.

According to the patent document 1, stress exerted to a gate electrodeis applied to a channel region of a silicon substrate. As a result,channel characteristic of a MOS transistor is improved without the needof preparing a strained silicon substrate.

As discussed, tensile stress exerted on a channel region causes increasein driving current of an nMOS transistor, while causing reduction indriving current in a pMOS transistor. In contrast, compressive stressexerted on the channel region causes increase in driving current of thepMOS transistor, while causing reduction in driving current of the nMOStransistor. Therefore, stress to be exerted should differ at leastbetween the nMOS and pMOS transistors.

It is thus required in the patent document 1 to employ different gateelectrode materials and different deposition temperatures thereofbetween the nMOS and pMOS transistors. As a result, a gate electrode ofthe nMOS transistor and that of the pMOS transistor cannot be providedin the same process step, causing complication of the manufacturingsteps.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide asemiconductor device allowing improvement in carrier mobility byapplying tensile stress only to a channel region of a desired MOStransistor, and allowing simplification of manufacturing steps. It isstill an object of the present invention to provide a method of thissemiconductor device.

According to a first aspect of the present invention, the semiconductordevice includes a polysilicon gate electrode provided on a siliconsubstrate. The gate electrode is subjected to compressive stress asinternal stress therein, to apply tensile stress to the siliconsubstrate. Ions having a mass number of 70 or more are implanted intothe gate electrode.

Tensile stress is applied to a region in the silicon substrate definedunder a predetermined gate electrode. Therefore, lattice spacing iswidened in this region of the silicon substrate. When this gateelectrode is applied to an nMOS transistor, for example, enhancement ofcarrier mobility can be provided, thus contributing to performanceimprovement of the nMOS transistor.

According to a second aspect of the present invention, the method ofmanufacturing a semiconductor device includes the following steps (a)through (d). In the step (a), a non-single crystalline silicon gateelectrode is provided on a silicon substrate. In the step (b), ionshaving a mass number of 70 or more are implanted into the gateelectrode. In the step (c), a predetermined film is deposited at atemperature of 550.degree. C. or less, to cover the gate electrodeincluding therein the ions having a mass number of 70 or more. In thestep (d), thermal processing is performed at a temperature of more than550.degree. C. while covering the gate electrode with the predeterminedfilm.

Compressive residual stress is exerted on a predetermined gate electrodeas internal stress therein, to apply tensile stress to a region in thesilicon substrate defined under the gate electrode. Therefore, latticespacing is widened in this region of the silicon substrate. When thisgate electrode is applied to an nMOS transistor, for example,enhancement of carrier mobility can be provided, thus contributing toperformance improvement of the nMOS transistor. Further, a mass numberof ion to be implanted may vary according to the type of the gateelectrode, or a part of the predetermined film on the predetermined gateelectrode may be removed prior to thermal processing. Therefore, evenwhen a plurality of gate electrodes are formed in the same step, only adesired gate electrode can easily be subjected to high compressivestress exerted thereon. As a result, simplification of the manufacturingsteps is provided.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 9 illustrate steps of manufacturing a semiconductordevice according to a first preferred embodiment of the presentinvention;

FIGS. 10A and 10B illustrate stress distribution in a direction ofchannel length in an nMOS transistor of the semiconductor deviceaccording to the first preferred embodiment of the present invention,and in the nMOS transistor of the background-art semiconductor device,respectively;

FIG. 11 illustrates a step of manufacturing a semiconductor deviceaccording to a second preferred embodiment of the present invention;

FIG. 12 illustrate a step of manufacturing a semiconductor deviceaccording to a third preferred embodiment of the present invention;

FIGS. 13 through 16 illustrate steps of manufacturing a semiconductordevice according to a fourth preferred embodiment of the presentinvention;

FIGS. 17 through 20 illustrate steps of manufacturing a semiconductordevice according to a fifth preferred embodiment of the presentinvention; and

FIG. 21 illustrates a step of manufacturing a semiconductor deviceaccording to a sixth preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Preferred Embodiment

The present inventors have found that when amorphous silicon including alarge number of ions implanted therein undergoes thermal processing toform recrystallized polycrystalline silicon (polysilicon), volumeexpansion of silicon occurs. It has also been found that the amount ofexpansion of silicon is controlled largely by the mass number of ionsimplanted therein, and more particularly, that the amount of expansionincreases as the mass number of implanted ions becomes larger (morespecifically, 70 or more). It has been confirmed as well that the amountof expansion increases as the implant dose of ions increases.

FIGS. 1 through 9 illustrate steps of manufacturing a semiconductordevice according to the first preferred embodiment of the presentinvention. In each one of FIGS. 1 through 9, the left half shows aregion for forming an nMOS transistor (hereinafter called as an “nMOSregion”), and the right half shows a region for forming a pMOStransistor (hereinafter called as a “pMOS region”).

First, an element isolation film 11, a p-well 12, and an n-well 22 areprovided on a silicon substrate 10 by the conventional technique.Thereafter a silicon oxide film 31 as a gate insulating film is providedthereon. Subsequently, a silicon film 32 for forming gate electrodes isprovided on the silicon oxide film 31 (FIG. 1). The silicon film 32 isnot of a single crystalline structure, but either of an amorphous orpolycrystalline structure.

Next, the non-single crystalline silicon film 32 is patterned byphotolithography, to form gate electrodes 14 and 24 on the p-well 12 andon the n-well 22, respectively (FIG. 2).

Thereafter, a resist mask 33 is formed by photolithography to have anopening in the pMOS region. Using the resist mask 33 and the gateelectrode 24 as a mask, p-type dopants having a relatively small massnumber such as B ions are then implanted, whereby a p-type source/drainextension layer 26 a is formed in the n-well 22 at a relatively shallowdepth (FIG. 3). At this time, ions are also implanted into the gateelectrode 24, and therefore, non-single crystalline silicon forming thegate electrode 24 is partially amorphized. However, the degree ofamorphization thus caused is low due to the relatively small mass numberof the implanted ions.

Next, a resist mask 34 is formed by photolithography to have an openingin the nMOS region. Using the resist mask 34 and the gate electrode 14as a mask, n-type dopants having a relatively large mass number (70 ormore) such as As ions or Sb ions are then implanted, whereby an n-typesource/drain extension layer 16 a is formed in the p-well 12 at arelatively shallow depth (FIG. 4). At this time, ions of a relativelylarge mass number are also implanted into the gate electrode 14.Accordingly, non-single crystalline silicon forming the gate electrode14 is partially amorphized.

Thereafter, sidewalls 15 and 25 are provided on the respective sidesurfaces of the gate electrodes 14 and 24. Further, the silicon oxidefilm 31 is etched, to form gate insulating films 13 and 23 under thegate electrodes 14 and 24, respectively (FIG. 5). The depositiontemperature for the sidewalls 15 and 25 is set to be less than the oneat which recrystallization of silicon starts (about 550° C.).

Subsequently, another resist mask 35 is formed to have an opening in thepMOS region. Using the resist mask 35, the gate electrode 24 and thesidewall 25 as a mask, p-type dopants having a relatively small massnumber such as B ions are then implanted with a dose of 4×10¹⁵/cm² ormore, whereby a p-type source/drain diffusion layer 26 b is formed inthe n-well 22 at a relatively great depth (FIG. 6). The p-typesource/drain extension layer 26 a and the p-type source/drain diffusionlayer 26 b form a p-type source/drain region 26. At this time, ions arealso implanted into the gate electrode 24, and therefore, non-singlecrystalline silicon forming the gate electrode 24 is partiallyamorphized. However, the degree of amorphization thus caused is low dueto the relatively small mass number of the implanted ions.

Thereafter, a resist mask 36 is formed to have an opening in the nMOSregion. Using the resist mask 36, the gate electrode 14 and the sidewall15 as a mask, n-type dopants having a relatively large mass number suchas As ions or Sb ions are then implanted with a dose of 4×10¹⁵/cm² ormore, whereby an n-type source/drain diffusion layer 16 b is formed inthe p-well 12 at a relatively great depth (FIG. 7). The n-typesource/drain extension layer 16 a and the n-type source/drain diffusionlayer 16 b form an n-type source/drain region 16. At this time, ions ofa relatively large mass number are also implanted into the gateelectrode 14, and therefore, amorphization of non-single crystallinesilicon forming the gate electrode 14 further proceeds.

Subsequently, a silicon oxide film 40 is provided on the gate electrodes14 and 24, and on the sidewalls 15 and 25 at a temperature which is lessthan the one at which recrystallization of silicon starts (about 550°C.) (FIG. 8).

Next, thermal processing such as RTA (rapid thermal annealing) isperformed on the silicon oxide film 40 covering the gate electrodes 14and 24, and the sidewalls 15 and 25, at a temperature ranging betweenabout 950° C. and 1100° C. The period of this thermal processing may be30 seconds or less including momentary annealing (spike annealing). As aresult, damage caused by ion irradiation is repaired, and the dopantsare activated. At the same time, recrystallization of amorphous siliconoccurs, whereby the gate electrodes 14 and 24 turn into polysilicon.

Those ions of a relatively large mass number (70 or more) such as Asions or Sb ions are implanted in large quantity into the gate electrode14 of the nMOS transistor. Hence, the gate electrode 14 tends to expand.However, as the surfaces of the gate electrode 14 and the sidewall 15are covered with the silicon oxide film 40, the gate electrode 14 canhardly expand. The tendency of the gate electrode 14 to expand causeshigh compressive residual stress as internal stress therein, thusapplying tensile stress to a channel region defined under the gateelectrode 14.

In contrast, only those ions of a relatively small mass number areimplanted into the gate electrode 24 of the pMOS transistor. Hence, thegate electrode 24 scarcely tends to expand, whereby little residualstress is exerted on the gate electrode 24. As a result, there occurssubstantially no stress to be applied to a channel region defined underthe gate electrode 24.

As an exemplary way of silicidation of the gate electrodes 14 and 24,and the upper portions of the source/drain regions 16 and 26, thesilicon oxide film 40 is removed, and thereafter, a metal film such asCo film is entirely deposited by sputtering. Next, thermal processing isperformed at a relatively low temperature of about 350° C. to 550.° C.,whereby this metal film is reacted with the gate electrodes 14 and 24,and with the source/drain regions 16 and 26. Then, unreacted metal filmremaining on the element isolation film 11, and on the sidewalls 15, 25is selectively removed, followed by high-temperature thermal processing.As a result, silicide layers 14 a and 24 a are formed in the upperportions of the gate electrodes 14 and 24, respectively, and silicidelayers 16 c and 26 c are formed in the upper portions of thesource/drain regions 16 and 26, respectively (FIG. 9).

Thereafter, predetermined device elements including interlayerinsulating film, contact, interconnect line, and the like, are provided,which is the completion of the manufacturing process of thesemiconductor device.

FIG. 10A illustrates stress distribution in cross section in a directionof channel length in the nMOS transistor according to the firstpreferred embodiment, and FIG. 10B illustrates stress distribution incross section in a direction of channel length of the background-artnMOS transistor, namely, the transistor including the gate electrode 14into which only ions of a relatively small mass number are implanted. Itis seen that in the nMOS transistor of the first preferred embodiment,high compressive residual stress is exerted on the gate electrode 14,and tensile stress is applied to the channel region. Therefore, thelattice spacing of silicon in the channel region of the nMOS transistoris widened and carrier mobility is enhanced, resulting in improvement incharacteristic of the MOS transistor.

In contrast, little residual stress is exerted on the gate electrode 24of the pMOS transistor, and thus there occurs substantially no stress tobe applied to the channel region under the gate electrode 24. Asdiscussed, application of tensile stress to the channel region of thepMOS transistor provides no effectiveness, as it causes reduction indrain current of the pMOS transistor. That is, in the device includingboth pMOS and NMOS transistors, tensile stress is preferably be appliedonly to the channel region of the nMOS transistor. In the firstpreferred embodiment, no tensile stress is applied to the channel regionof the pMOS transistor. As a result, while suppressing reduction indrain current of the pMOS transistor, improvement in characteristic ofthe nMOS transistor is realized.

In addition, in the first preferred embodiment, ions (for formation ofthe n-type source/drain region 16) to be implanted into the gateelectrode 14 have a relatively large mass number, and ions (forformation of the p-type source/drain region 26) to be implanted into thegate electrode 24 have a relatively small mass number. As a result, onlythe channel region of the nMOS transistor undergoes high tensile stressapplied thereto. That is, even when the gate electrodes 14 and 24 areformed in the same step, as the n-type dopants and the p-type dopants tobe implanted in the subsequent step have different mass numbers, thedegrees of stress to be applied to the respective channel regions of thenMOS and pMOS transistors can vary therebetween. Namely, formation ofthe nMOS and pMOS transistors does not require separate steps, allowingsimplification of the manufacturing steps.

As discussed, when amorphous silicon including a large number of ionsimplanted therein is recrystallized to turn into polysilicon, thetendency of silicon to expand increases as the mass number of theimplanted ions increases. Due to this, the greater the mass number ofthe implanted ions into the gate electrode 14, the higher thecompressive residual stress thereon. As a result, tensile stress to beapplied to the channel region becomes higher, providing enhancedeffectiveness of the first preferred embodiment. Further, thecompressive residual stress on the gate electrode 14 becomes higher asthe dose of the implanted ions implanted therein increases. In theforegoing discussion, the exemplary dose of ions for forming the n-typesource/drain diffusion layer 16 b is 4×10¹⁵/cm² or more, which is thestandard amount therefor. The applicability of the present invention isnot limited to this. The dose of approximately 4×10¹⁵/cm² is sufficientenough to maintain effectiveness of the present invention. On the otherhand, increased effectiveness can be achieved by the greater amount. Thecompressive stress on the gate electrode 14 itself is generated with adose less than 4×10¹⁵/cm².

In the step of FIG. 8, the silicon oxide film 40 is provided as apredetermined film to be on the gate electrodes 14 and 24. As long asthe temperature for forming this film satisfies the condition that itshould be less than the one for starting recrystallization of silicon(about 550° C.), alternative materials may be used for thispredetermined film. As long as compressive residual stress is exerted onthe gate electrode 14, alternative films such as metal film, silicidefilm, or stacked film thereof, may be employed. In this case, aftercompressive residual stress is exerted on the gate electrode 14 bythermal processing, this alternative film is removed. Thereafter, aninsulating film such as silicon oxide film is further provided.

The compressive residual stress exerted on the gate electrode 14 alsobecomes greater when the predetermined film 40 has a property that itshrinks by the foregoing thermal processing (a silicon oxide film hassuch property). It is confirmed by the present inventors that the higherthe temperature for the thermal processing for recrystallization ofamorphous silicon, and the greater the thickness of the predeterminedfilm 40, the greater the compressive residual stress on the gateelectrode 14, whereby improved carrier mobility is provided.

In the first preferred embodiment, an ordinary silicon substrate is usedfor forming a MOS transistor. Alternatively, it may be a “strainedsilicon substrate” as discussed in the foregoing description of thebackground art. In this case, carrier mobility in the channel region ofthe nMOS transistor can be reliably improved to a greater degree.

Second Preferred Embodiment

As discussed in the first preferred embodiment, ions having a relativelysmall mass number are implanted into the gate electrode 24, andtherefore, there will be little residual stress to be applied to thegate electrode 24 even with the existence of the silicon oxide film 40thereon. In the case of ion implantation in large quantity, however,there may be some compressive residual stress to be exerted on the gateelectrode 24 even when the implanted ions have a small mass number.

The steps of manufacturing a semiconductor device according to thesecond preferred embodiment of the present invention will be given.First, following the same steps as those of the first preferredembodiment shown in FIGS. 1 through 8, the NMOS and pMOS transistors areprovided. Further provided thereon is the silicon oxide film 40. Next,the silicon oxide film 40 in the pMOS region is removed so that anopening is defined therein, as shown in FIG. 11.

Thermal processing is thereafter performed at a temperature rangingbetween about 950°. C. and 1100° C., to repair damage and to activatethe dopants. At the same time, recrystallization of amorphous siliconoccurs, whereby the gate electrodes 14 and 24 turn into polysilicon.

The surfaces of the gate electrode 14 and the sidewall 15 of the NMOStransistor are covered with the silicon oxide film 40. Therefore, thetendency of the gate electrode 14 to expand causes high compressiveresidual stress as internal stress therein, thus applying tensile stressto the channel region under the gate electrode 14.

On the other hand, the surfaces of the gate electrode 24 and thesidewall 25 of the pMOS transistor are exposed without being coveredwith the silicon oxide film 40. Accordingly, even when the gateelectrode 24 expands slightly, there will be little residual stress onthe gate electrode 24. As a result, as compared with the first preferredembodiment, tensile stress to be exerted on the channel region of thepMOS transistor can be suppressed to a greater degree.

Third Preferred Embodiment

In the first and second preferred embodiments, ion implantation forforming the n-type source/drain diffusion layer 16 b is also operativeto implant ions into the gate electrode 14 for expansion thereof.Alternatively, ion implantation step into the gate electrode 14 may beperformed separately from the implantation step for formation of then-type source/drain diffusion layer 16 b.

The steps of manufacturing a semiconductor device according to the thirdpreferred embodiment of the present invention will be given. First,following the same steps as those of the first preferred embodimentshown in FIGS. 1 through 6, the NMOS and pMOS transistors are provided.Next, a resist mask 36 is formed to have an opening in the pMOS region.Thereafter, prior to formation of the n-type source/drain diffusionlayer 16 b in the nMOS region, electrically inactive ions having arelatively large mass number (70 or more) such as Ge ions are entirelyimplanted with a dose of 4×10¹⁵/cm² or more, as shown in FIG. 12. Atthis time, ions are implanted into the source/drain region of the nMOStransistor as well as into the gate electrode 14. Those ions implantedin this step are electrically inactive, and therefore, they are notoperative to serve as dopants.

Thereafter, as shown in FIG. 7, n-type dopants are implanted to form then-type source/drain diffusion layer 16 b. The n-type dopants to beimplanted in this step may be those having a relatively small massnumber such as P ions.

The subsequent steps are the same as those of the first preferredembodiment. Namely, the silicon oxide film 40 is provided on the gateelectrodes 14 and 24, and on the sidewalls 15 and 25, and thereafter,thermal processing is performed at a temperature ranging between about950° C. and 1100° C. As a result, recrystallization of amorphous siliconoccurs, whereby the gate electrodes 14 and 24 turn into polysilicon.

As mentioned, electrically inactive ions having a relatively large massnumber are implanted in large quantity into the gate electrode 14 of thenMOS transistor. Further, the surfaces of the gate electrode 14 and thesidewall 15 are covered with the silicon oxide film 40. Accordingly, thetendency of the gate electrode 14 to expand causes compressive residualstress as internal stress therein, thus applying tensile stress to thechannel region under the gate electrode 14.

In contrast, only those ions of a relatively small mass number areimplanted into the gate electrode 24 of the pMOS transistor. Hence,little residual stress is exerted on the gate electrode 24. As a result,there occurs substantially no stress to be applied to the channel regionunder the gate electrode 24.

Fourth Preferred Embodiment

In a silicon substrate subjected to high stress applied thereto, crystaldefect generally occurs with high probability. In a transistor providedon the silicon substrate having crystal defect, increase in leakagecurrent such as junction leakage current, gate current, or subthresholdleakage current, may occur. That is, the nMOS transistor of the presentinvention may encounter the problem of crystal defect resulting fromtensile stress applied to the channel region. Therefore, the nMOStransistor of the present invention may suffer from an increased amountof leakage current as compared with the one in the background art.

For example, a logic section of an ordinary semiconductor device isintended mainly for high-speed operation and response, giving priorityto high-speed operation even with the existence of some leakage current.On the other hand, in a memory section of an SRAM or DRAM, or in a logicsection of an LSI for mobile communication system, for example, evenslight increase in power consumption resulting from leakage currentshould be controlled. In view of this, the MOS transistor according tothe present invention may provide effectiveness for a circuit sectionplacing priority to high-speed operation (hereinafter referred to as a“high-speed circuit section”), while it may be unsuitable for a circuitsection requiring suppression of power consumption (hereinafter referredto as a “low-power circuit section”). In other words, it is preferableto use the MOS transistor of the present invention only in thehigh-speed circuit section of a semiconductor device, and to use theconventional MOS transistor in the low-power circuit section.

FIGS. 13 through 16 illustrate steps of manufacturing a semiconductordevice according to the fourth preferred embodiment of the presentinvention. As indicated in FIG. 13, in each one of FIGS. 13 through 16,the left half shows a high-speed circuit section placing priority tohigh-speed operation, and the right half shows a low-power circuitsection requiring suppression of power consumption. Assuming that thehigh-speed circuit section and the low-power circuit section eachcomprise an nMOS region for forming an nMOS transistor and a pMOS regionfor forming a pMOS transistor, the steps of manufacturing asemiconductor device according to the fourth preferred embodiment willbe given.

First, an element isolation film 11, p-wells 12 and 52, and n-wells 22and 62 are provided on a silicon substrate 10 by the conventionaltechnique. Thereafter, following the same steps as those of the firstpreferred embodiment shown in FIGS. 1 through 5, a gate insulating film13, a gate electrode 14, a sidewall 15, and an n-type source/drainextension layer 16 a are provided in the nMOS region of the high-speedcircuit section. Provided in the pMOS region of the high-speed circuitsection are a gate insulating film 23, a gate electrode 24, a sidewall25, and a p-type source/drain extension layer 26 a. Provided in the nMOSregion of the low-power circuit section are a gate insulating film 53, agate electrode 54, a sidewall 55, and an n-type source/drain extensionlayer 56 a. Further, a gate insulating film 63, a gate electrode 64, asidewall 65, and a p-type source/drain extension layer 66 a are providedin the pMOS region of the low-power circuit section.

Subsequently, a resist mask 71 is formed to have an opening in therespective pMOS regions of the high-speed circuit section and thelow-power circuit section. Using the resist mask 71, the gate electrodes24 and 64, and the sidewalls 25 and 65 as a mask, p-type dopants havinga relatively small mass number such as B ions are then implanted with adose of 4×10¹⁵/cm² or more, whereby a p-type source/drain diffusionlayer 26 b and a p-type source/drain diffusion layer 66 b are formed(FIG. 13). The p-type source/drain extension layer 26 a and the p-typesource/drain diffusion layer 26 b form a p-type source/drain region 26,and the p-type source/drain extension layer 66 a and the p-typesource/drain diffusion layer 66 b form a source/drain region 66. At thistime, ions are also implanted into the gate electrodes 24 and 64.

Next, a resist mask 72 is formed to have an opening in the respectiveNMOS regions of the high-speed circuit section and the low-power circuitsection. Using the resist mask 72, the gate electrodes 14 and 54, andthe sidewalls 15 and 55 as a mask, n-type dopants having a relativelylarge mass number such as As ions or Sb ions are implanted, whereby ann-type source/drain diffusion layer 16 b and an n-type source/draindiffusion layer 56 b are formed (FIG. 14). The n-type source/drainextension layer 16 a and the n-type source/drain diffusion layer 16 bform an n-type source/drain region 16, and the n-type source/drainextension layer 56 a and the n-type source/drain diffusion layer 56 bform an n-type source/drain region 56. At this time, ions are alsoimplanted into the gate electrodes 14 and 54.

Next, a silicon oxide film 80 is provided to cover the high-speedcircuit section while exposing the low-power circuit section. Moreparticularly, the silicon oxide film 80 is provided on the gateelectrodes 14 and 24, and on the sidewalls 15 and 25 of the high-speedcircuit section, at a temperature which is less than the one at whichrecrystallization of silicon starts (about 550° C.) (FIG. 15).

Thereafter, thermal processing is performed on the silicon oxide film 80covering the gate electrodes 14 and 24, and the sidewalls 15 and 25, ata temperature ranging between about 950° C. and 1100°. C., causingrecrystallization of amorphous silicon. As a result, the gate electrodes14, 24, 54 and 64 turn into polysilicon.

Those ions of a relatively large mass number (70 or more) such as Asions or Sb ions are implanted in large quantity into the gate electrode14 of the nMOS transistor of the high-speed circuit section. Further,the surfaces of the gate electrode 14 and the sidewall 15 are coveredwith the silicon oxide film 80. As a result, high compressive residualstress is exerted as internal stress on the gate electrode 14, thusapplying tensile stress to the channel region under the gate electrode14.

Those ions of a relatively large mass number such as As ions or Sb ionsare further implanted in large quantity into the gate electrode 54 ofthe nMOS transistor of the low-power circuit section. However, thesurfaces of the gate electrode 54 and the sidewall 55 are exposed,whereby little residual stress is exerted on the gate electrode 54. As aresult, there occurs substantially no stress to be applied to thechannel region under the gate electrode 54.

Only those ions of a relatively small mass number are implanted into thegate electrodes 24 and 64 of the respective pMOS transistors of thehigh-speed circuit section and the low-power circuit section, andtherefore, little residual stress is exerted on the gate electrodes 24and 64. As a result, there occurs substantially no stress to be appliedto the respective channel regions under the gate electrodes 24 and 64.

As discussed, according to the manufacturing steps of the fourthpreferred embodiment, high tensile stress is applied only to the channelregion of the NMOS transistor of the high-speed circuit section, thusallowing performance improvement. Further, substantially no stress isapplied to the channel region of the pMOS transistor of the high-speedcircuit section, and to the respective channel regions of the pMOS andnMOS transistors of the low-power circuit section, whereby increase inleakage current resulting from crystal defect can be controlled.

As an exemplary way of silicidation of the gate electrode andsource/drain region of a certain MOS transistor, a metal film such as Cofilm is deposited by sputtering, followed by thermal processing at arelatively low temperature ranging between 350° C. and 550° C., wherebythe metal film and silicon are reacted. Then unreacted metal filmremaining on the insulating film is selectively removed. Thereafterhigh-temperature thermal processing follows.

In an LSI for mobile communication system, for example, silicidation ofthe gate electrode and source/drain region of the MOS transistor of thelow-power circuit section is required in many cases. For suchsilicidation, the silicon oxide film 80 provided in the foregoing stepto cover the high-speed circuit section while exposing the low-powercircuit section may be further operative to serve as a mask. Aftersilicidation using the silicon oxide film 80 as a mask, silicide layers54 a and 64 a are formed in the upper portions of the gate electrodes 54and 64 of the low-power circuit section, respectively, and silicidelayers 56 c and 66 c are formed in the upper portions of thesource/drain regions 56 and 66 of the low-power circuit section,respectively.

According to the fourth preferred embodiment, for recrystallization ofamorphous silicon, the silicon oxide film 80 has such a shape that thelow-power circuit section is exposed. Therefore, application of hightensile stress can be limited to the channel region of the nMOStransistor of the high-speed circuit section. That is, even when thegate electrodes 14, 24, 54 and 64 are all provided in the same step,only a channel region of a certain nMOS transistor can be subjected tohigh tensile stress applied thereto. As a result, simplification of themanufacturing steps is realized.

Fifth Preferred Embodiment

Similar to the fourth preferred embodiment, in the fifth preferredembodiment of the present invention, the MOS transistor according to thepresent invention is applied only to a high-speed section of asemiconductor device, while the conventional MOS transistor is appliedto a low-power section thereof.

FIGS. 17 through 20 illustrate steps of manufacturing a semiconductordevice according to the fifth preferred embodiment. Similar to FIG. 13,in each one of FIGS. 17 through 20, the left half shows a high-speedcircuit section placing priority to high-speed operation, and the righthalf shows a low-power circuit section requiring suppression of powerconsumption. The high-speed circuit section and the low-power circuitsection each comprise nMOS and pMOS regions.

The steps of manufacturing the semiconductor device according to thefifth preferred embodiment will be described with reference to FIGS. 17through 20. First, similar to the fourth preferred embodiment, anelement isolation film 11, p-wells 12 and 52, and n-wells 22 and 62 areprovided on a silicon substrate 10. Thereafter, a gate insulating film13, a gate electrode 14, a sidewall 15, and an n-type source/drainextension layer 16 a are provided in the nMOS region of the high-speedcircuit section. Provided in the pMOS region of the high-speed circuitsection are a gate insulating film 23, a gate electrode 24, a sidewall25, and a p-type source/drain extension layer 26 a. Provided in the nMOSregion of the low-power circuit section are a gate insulating film 53, agate electrode 54, a sidewall 55, and an n-type source/drain extensionlayer 56 a. Further, a gate insulating film 63, a gate electrode 64, asidewall 65, and a p-type source/drain extension layer 66 a are providedin the pMOS region of the low-power circuit section. In the fifthpreferred embodiment, for formation of the n-type source/drain extensionlayers 16 a and 56 a, n-type dopants having a relatively small massnumber such as P ions are implanted.

Subsequently, a resist mask 71 is formed to have an opening in therespective pMOS regions of the high-speed circuit section and thelow-power circuit section. Using the resist mask 71, the gate electrodes24 and 64, and the sidewalls 25 and 65 as a mask, p-type dopants havinga relatively small mass number such as B ions are then implanted with adose of 4×10¹⁵/cm² or more, whereby a p-type source/drain diffusionlayer 26 b and a p-type source/drain diffusion layer 66 b are formed(FIG. 13). The p-type source/drain extension layer 26 a and the p-typesource/drain diffusion layer 26 b form a p-type source/drain region 26,and the p-type source/drain extension layer 66 a and the p-typesource/drain diffusion layer 66 b form a source/drain region 66. At thistime, ions are also implanted into the gate electrodes 24 and 64.

Next, a resist mask 73 is formed to have an opening in the nMOS regionof the low-power circuit section. Using the resist mask 73, the gateelectrode 54 and the sidewall 55 as a mask, n-type dopants having arelatively small mass number such as P ions are implanted, whereby ann-type source/drain diffusion layer 56 b is formed (FIG. 17). The n-typesource/drain extension layer 56 a and the n-type source/drain diffusionlayer 56 b form an n-type source/drain region 56. At this time, ions arealso implanted into the gate electrode 54.

Thereafter, a resist mask 74 is formed to have an opening in the nMOSregion of the high-speed circuit section. Using the resist mask 74, thegate electrode 14 and the sidewall 15 as a mask, n-type dopants having arelatively large mass number such as As ions or Sb ions are implanted,whereby an n-type source/drain diffusion layer 16 b is formed (FIG. 18).The n-type source/drain extension layer 16 a and the n-type source/draindiffusion layer 16 b form an n-type source/drain region 16. At thistime, ions are also implanted into the gate electrode 14.

Next, a silicon oxide film 81 is provided to cover the high-speedcircuit section and the low-power circuit section. More particularly,the silicon oxide film 81 is provided on the gate electrodes 14, 24, 54and 64, and on the sidewalls 15, 25, 55 and 65, at a temperature whichis less than the one at which recrystallization of silicon starts (about550° C.) (FIG. 19).

Thereafter, thermal processing is performed on the silicon oxide film 81covering the gate electrodes 14, 24, 54 and 64, and the sidewalls 15,25, 55 and 65, at a temperature ranging between about 950° C. and 1100°C., causing recrystallization of amorphous silicon. As a result, thegate electrodes 14, 24, 54 and 64 turn into polysilicon.

Those ions of a relatively large mass number (70 or more) such as Asions or Sb ions are implanted into the gate electrode 14 of the nMOStransistor of the high-speed circuit section. Further, the surfaces ofthe gate electrode 14 and the sidewall 15 are covered with the siliconoxide film 81. As a result, high compressive residual stress is exertedas internal stress on the gate electrode 14, thus applying tensilestress to the channel region under the gate electrode 14.

Only those ions of a relatively small mass number are implanted into thegate electrode 54 of the nMOS transistor of the low-power circuitsection, whereby little residual stress is exerted on the gate electrode54. As a result, there occurs substantially no stress to be applied tothe channel region under the gate electrode 54.

Only those ions of a relatively small mass number are further implantedinto the gate electrodes 24 and 64 of the respective pMOS transistors ofthe high-speed circuit section and the low-power circuit section, andtherefore, little residual stress is exerted on the gate electrodes 24and 64. As a result, there occurs substantially no stress to be appliedto the respective channel regions under the gate electrodes 24 and 64.

As discussed, according to the manufacturing steps of the fifthpreferred embodiment, high tensile stress is applied only to the channelregion of the NMOS transistor of the high-speed circuit section, thusallowing performance improvement. Further, substantially no stress isapplied to the channel region of the pMOS transistor of the high-speedcircuit section, and to the respective channel regions of the pMOS andnMOS transistors of the low-power circuit section, whereby increase inleakage current resulting from crystal defect can be controlled.

In an LSI for mobile communication system, for example, for silicidationof the gate electrode and source/drain region of the MOS transistor ofthe low-power circuit section, an opening is defined in the siliconoxide film 81 to expose the low-power circuit section. Using thissilicon oxide film 81 as a mask, silicidation is performed. As a result,silicide layers 54 a and 64 a are formed in the upper portions of thegate electrodes 54 and 64 of the low-power circuit section,respectively, and silicide layers 56 c and 66 c are formed in the upperportions of the source/drain regions 56 and 66 of the low-power circuitsection, respectively.

According to the fifth preferred embodiment, n-type dopants to beimplanted into the high-speed circuit section and into the low-powercircuit section have different mass numbers therebetween. Therefore,application of high tensile stress can be limited to the channel regionof the nMOS transistor of the high-speed circuit section. That is, evenwhen the gate electrodes 14, 24, 54 and 64 are all provided in the samestep, only a channel region of a certain nMOS transistor can besubjected to high tensile stress applied thereto.

Sixth Preferred Embodiment

As discussed, the nMOS transistor according to the present invention islikely to suffer from crystal defect, which may result in leakagecurrent such as junction leakage current, gate current, or subthresholdleakage current of the MOS transistor. That is, the nMOS transistor ofthe present invention will suffer from an increased amount of leakagecurrent as compared with the one in the background art. The sixthpreferred embodiment of the present invention is directed to solve thisproblem.

By way of example, after the gate electrodes 14 and 24 are provided asin FIG. 2 following the same step as that of the first preferredembodiment, the surfaces of the gate electrodes 14 and 24, and of thesilicon substrate 10 are oxidized. More precisely, the silicon oxidefilm 31 remains on the silicon substrate 10 at this time, and therefore,it is reoxidized. The resultant structure is as given in FIG. 21, inwhich a silicon oxide film 90 is provided on the surfaces of the gateelectrodes 14 and 24, and bird's beaks 90 a are defined at respectiveedge portions of the gate electrodes 14 and 24. The subsequent steps arethe same as those of FIGS. 3 through 9, and therefore, the descriptionthereof is omitted.

Resulting from the existence of the bird's beaks 90 a, the insulatingfilm has a larger thickness at the edge portions of the gate electrodes14 and 24, providing suppression of tunneling current and relaxation ofgate electric field. As a result, subthreshold leakage current can bereduced, allowing suppression of increase in leakage current of the MOStransistor. For this reason, the sixth preferred embodiment iseffectively applied to the MOS transistor having a probability ofincrease in leakage current, especially to the nMOS transistor of thepresent invention subjected to tensile stress applied to its channelregion.

Further, as the gate electrodes 14 and 24 remain covered with thesilicon oxide film 90 during thermal processing to be performed in thesubsequent step, variation in resistance value of the polysilicon gateelectrodes can be controlled in the non-silicided region.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

The invention claimed is:
 1. A semiconductor device comprising: a semiconductor substrate including a first region and a second region; a first gate insulating film of a first NMOS transistor formed over the first region; a first gate electrode of the first NNOS transistor formed over the first gate insulating film; a first source region and a first drain region of the first NMOS transistor formed in the first region; a first channel region of the first NMOS transistor formed in the first region, formed under the first gate electrode and formed between the first source region and the first drain region; a second gate insulating film of a second NMOS transistor formed over the second region; a second gate electrode of the second NNOS transistor formed over the second gate insulating film; a second source region and a second drain region of the second NMOS transistor formed in the second region; a second channel region of the second NMOS transistor formed in the second region, formed under the second gate electrode and formed between the second source region and the second drain region; wherein a first tensile stress is applied in the first channel region of the first NMOS transistor by the first gate electrode, wherein a second tensile stress is applied in the second channel region of the second NMOS transistor by the second gate electrode, and wherein the first tensile stress is larger than the second tensile stress.
 2. A semiconductor device according to the claim 1, wherein a drain current of the first NMOS transistor is improved by the first tensile stress.
 3. A semiconductor device according to the claim 2, wherein the first NMOS transistor is used for a memory circuit.
 4. A semiconductor device according to the claim 3, wherein the memory circuit is a SRAM or a DRAM.
 5. A semiconductor device according to the claim 4, wherein the first and second gate electrodes are formed of a silicon film, respectively.
 6. A semiconductor device according to the claim 1, wherein the first and second gate electrodes are formed of a silicon film, respectively. 